1. Field of the Invention
The present invention relates to a fluid-filled elastic cushioning device used on a motor vehicle, for example, for elastically connecting a shaft or rod member to a support structure, such that the cushioning device damps or isolates input vibrations applied thereto in its axial direction, in particular.
2. Discussion of the Prior Art
As a cushioning device used for a suspension system of a motor vehicle, there is known a device which elastically or flexibly connects a shaft or rod to a support structure, in such a manner as to damp or isolate primarily the vibrations applied in the axial direction of the device. For example, such a cushioning device is used as a body mount, cab mount, or a member mount or a strut bar cushion of the motor vehicle.
To improve the vibration damping and/or isolating capability of such a cushioning device, there have been recently proposed various types of fluid-filled elastic cushions or mounts, wherein an elastic body has a plurality of fluid chambers which are filled with a non-compressible fluid and which communicate with each other through an orifice passage or passages. The vibrations applied to the cushion or mount are damped due to a flow resistance of the fluid through the orifice passages, or based on resonance of the fluid masses in the passages.
The known types of fluid-filled elastic cushions or mounts exhibit a linear load-strain relationship. Namely, the spring characteristic of the cushions or mounts are such that the amount of elastic deformation of the elastic body is changed in substantially linear relationship with the magnitude of the input vibrational load. In the known fluid-filled elastic cushioning devices, no provisions are made for giving a non-linear load-strain spring characteristic.
In the fluid-filled elastic cushioning devices of the known types, the elastic body should have a relatively high degree of stiffness or hard spring characteristic, in order to resist an input vibrational load having a large amplitude, or to avoid an excessive amount of elastic deformation of the elastic body. The relatively high stiffness of the elastic body means a relatively small amount of change in the volumes of the fluid chambers upon application of vibrations of ordinary amplitudes, i.e., a relatively small amount of flow of the fluid through the orifice passage or passages between the fluid chambers. Consequently, the relatively stiff or hard elastic body of the cushioning device results in insufficient vibration damping and/or isolating capability based on the fluid flow resistance or resonance of the fluid masses.
Where the known fluid-filled elastic cushioning device is utilized as a strut bar cushion in a suspension system of an automotive vehicle, the elastic body should exhibit comparatively stiff spring characteristic, in order to reliably deal with a high vibrational load. However, this stiff spring characteristic is undesirable for dealing with harshness shocks which may occur during running of the vehicle on a stepped road surface, because of the reduced vibration damping effect based on the fluid flows through the orifice passages. Hence, the known fluid-filled elastic cushioning devices fail to assure satisfactory driving comfort of the vehicle.